During periadolescence, mammals display specific patterns of social behavior, termed social play behavior or rough-and-tumble play. Social play is a natural reinforcer, and it is thought that social play is of major importance for social and cognitive development. Despite its importance, the underlying neurobiological mechanisms of social play are largely unknown. Because social play is a reinforcer, it is likely that the neural substrates underlying natural and drug reinforcement also play a prominent role in the regulation of social play behavior. Interestingly, periadolescence, the period in life that social play is most abundant, roughly corresponds to the age that individuals will start experimenting with drugs of abuse. Thus, the study of the effects of drugs of abuse on social play behavior can serve two purposes: it will help to elucidate the neural substrates of social play behavior and it will show how periadolecent drug use impacts on social interactions and hence, social development. With regard to the relation between social behavior and drug abuse, there is a wealth of evidence that antisocial personality traits predispose to the development of drug abuse. There have not been many animal studies to investigate this topic. Therefore, the present proposal aims to investigate 1. The effects of drugs of abuse on social play behavior in periadolescent rats, 2. The neuropharmacology of the rewarding and motivational aspects of social play in rats and 3. Whether individual social characteristics of rats can predict subsequent drug self-administration and impulse control. The present proposal aims to study the brain processes involved in social behavior in young rats and the effects of drug use on social interactions in young rats. By changing the quality of social interactions, drugs of abuse can have a major impact on the development of brain and behavior. We will also study whether individual differences in social behavior can predict the vulnerability to subsequent drug abuse, thus allowing for animal research to investigate the neural and behavioral underpinnings of the well-known predictive value of antisocial personality traits for later drug abuse.